A vent is essential if you are firing in an attached garage or other area connected to a living space. The fumes from firing are not pleasant or safe. Even if your kiln is in a separate area, there are several reasons why you might want a vent.
1. You tend to get better glaze colors and fewer glaze defects. Vents help even out the temperature, bring a lot of oxygen to encourage good color development and, keep colors from affecting nearby colors.
2. If you fire without a vent, you have to prop the lid for the first few hours of a bisque firing. For many people, that kind of defeats the purpose of having an electronic controller to provide is "automatic firing." If you use a vent, you don't have to prop the lid. You just turn the vent on when you turn the kiln on, and leave it on for the full firing.
3. Elements last longer.
A vent is pretty easy to install. You attach dryer ducting to the vent unit, and run that outside of your house similar to running dryer ducting. If desired you can make a hole through an exterior wall, just like your dryer has. Or if you don't want to cut through your exterior wall, you can run ducting out a window or a door with good results. A vent can be added to the kiln later if you don't wish to purchase it with your kiln.